Molybdenum coated with heat-resistant alloys by casting



Reaper DA j .ETAL 3,336,120 MOLYBDENLYJM COATED WITH3HEATBESISTANTIALL'QYS BY CASTING Aug. 15, 1967 2 Sheets-Sheet 2 Filed Feb.

I 70st 20m. 70100 0 41 0 A N AWE w m INVENTORS Hrai nkaslwi Renpai- /a vBYJW MW ATTORNEYS .Ue red O 3,336,120 MOLYBDENUM COATED WITHHEAT-RESISTANT ALLOYS BY CASTING Renpei Yoda, Tokyo, and Takashi Arai,Kawaguchi-shi,

Japan, assignors to Director of National Research Institute of Metals,Tokyo, Japan Filed Feb. 3, 1965, Ser. No. 430,047 Claims priority,application Japan, Sept. 19, 1964, 39/53,683 3 Claims. (Cl. 29-198) Thisinvention relates to molybdenum coated with heatresistant alloys bycasting. More particularly, this invention relates to molybdenum coveredwith heat-resistant alloys by applying casting and cladding.

Refractory metals having melting point much higher than iron canmaintain their strength up to a high temperature but since they havedrawback of being readily oxidized, they cannot be used in theatmosphere of air at a high temperature under the influence of highstress. Accordingly various protective coating methods have beenheretofore proposed but they are not safe enough unless their coveringlayers are sufficiently strong. Since even a slightest extent of crackformed in the covering during the time of their service at a hightemperature inevitably causes the ruin of the material.

Molybdenum is a representative refractory metal but it is completelylacking in anti-oxidation property. As protective coating methodsheretofore applied to molybdenum, electroplating, vacuum evaporation,metallikon and cladding of oxidation-resistant metal or alloy, formationof stable refractory intermetallic compound on the surface, ceramiccoating and the like can be illustrated. The method which can providethe most superior properties among above-mentioned ones is themultilayer lamination of chromium and nickel or the protective coveringof MoSi utilizing the vapor deposition method.

Accordingly it is an object of the present invention to providemolybdenum covered with a heat-resistant alloy to which can readilyapply such heat treatment-working in the-atmosphere of air as has beendenied with prior coating treatments. It is another object of thepresent invention to provide molybdenum covered with heat-resistantalloys which can stand the high temperature, under the influence of highstress, in the atmosphere of air, for a long time.

These and other objects can be attained by the present invention.According to the present invention, molybdenum is covered with aheat-resistant alloy by applying simultaneously'the treatment of castingand cladding. In other words, commercially available pure round rods ofmolybdenum are covered by casting with Nimonic 90, a

superior nickel-base heat-resistant alloy capable of standing hotworking and subsequently subjected to forgeworking to produce theprotective coating.

In this case, the protective material of heat-resistant alloy is notnecessarily limited to nickel-base alloy. Ironbase and cobalt-basealloys mayalso be useful so long as they can stand hot working.

With reference to the accompanying drawings, the nature of the presentmolybdenum coated with heat-resistant alloy is more fully depicted.

'FIGURE 1 shows photographs of macrostructure of camlachie crampedmaterial and worked material thereof in the case when a polybdenum rodhaving a diameter of 3,336,120 Patented Aug. 15, 1967 5 mm. is used.Molybdenum in the central part and surrounding Nimonic alloy areperfectly jointed by fusion with each other. There is no chance of crackforming even when it is worked. It is observed that coarse columnarcrystal in the camlachie cramped material are changed into finestructure.

FIGURE 2 shows the hardness distribution in the crosssection ofcamlachie cramped material (such similar expressions will be usedhereinafter to mean material coated by casting) and worked materialthereof. It is seen from FIGURE '2 that the jointed part of molybdenumwith Nimonic 90 alloy by fusion has extremely high hardness and a phaseof brittle compound is formed there. The hardnesses of matrix ofmolybdenum and that of Nimonic 90 alloy are almost the same. This phaseof brittle compound consists principally of MoNi. Even when otherheat-resistant coating alloy is used, the phase of molybdenum alloybecomes also a brittle one possessing high hardness. In general, thecoating of molybdenum by means of the method other than cladding isreadily cracked. Moreover by the cladding method, such a perfectly thickfusionjointed layer between molybdenum and coating material cannot beobtained as is the case of the present invention. Furthermore the heattreatment-working cannot be carried out freely at the high temperaturein the atmosphere of air. On the other hand, the molybdenum coatedaccording to the present invention can be freely subjected to hotworking treatment at a high temperature in the atmosphere of air. Thereis no danger of crack forming during the time of working. This is due tothe fact that thick Nimonic 90 alloy outside the layer of brittlecompound perfectly prevents molybdenum from being oxidized and since itperforms a function of lubricant, the phase of brittle compound canfreely be deformed. This fact can also be understood from the phenomenonthat non-metallic brittle inclusions existing in steel ingots areelongated in the direction of milling in steel plates. The compositesmaterial thus obtained by coating firmly the surface of molybdenum witha heat-resistant alloy, can maintain, at a high temperature, under theinfluence of high stress, in the atmosphere of air and for a long time,the strength which is rather superior but not inferior to those ofmolybdenum heretofore obtained in the experiments carried out in vacuaor in the atmosphere of inert gas. This indicates the fact that themolybdenum produced according tothe present invention can maintain theperfect antioxidation 'property at a high temperaure under theinfluenceofhigh stress.

A more comprehensive understanding of the invention can be obtained byreferring to the following illustrative example which is not intended,however, to be unduly limitative of the invention.

Example 1 whereby camlachie cramped molybdenum rods having across-section indicated in FIGURE 1(a) were obtained. Resultingcamlachie cramped molybdenum were subor properties. When molybdenumdiffused into the coating layer and the latter became rich in molybdenumor cracks were formed in the layer of molybdenum alloy or coating layerduring the long time of service in the TABLE l.-VARIOUS HEAT-RESISTANTALLOYS USEFUL IN THE COATING OF MOLYBDENUM Alloy Chemical Ni Co Fe Cr MoW Cb Mn Si Al Ti composition Nimonic 90 N 1 base. Remainder 18 20 1. 28-816 00 base... Remainder 3 20 4 4 4 1 0.2 0.4 -20 Fe base 20 Remainder25 1. 5 1 0.2

These worked specimens were machined into the test 20 pieces forcreep-rupture testing having the gauge parameter of 6 mm. (in the caseof molybdenum rods having a diameter of 6 mm. the gauge parameter was 7mm.). Testing was carried out in the atmosphere of air, at a temperatureof 1000 C. and under the influence of 5 kg./ mm. stress, the result ofwhich is indicated in Table 2 and FIGURE 3.

atmosphere of air, at a high temperature and under the influence of highstress, the ruin of molybdenum perfectly lacking in anti-oxidationproperty surely occurs. Among various coating treatments applied tomolybdenum, the multiple lamination of chromium and nickel looked most 5promising. Also in this example, the coating with Nimonic 90; Ni-Cralloy showed the most advantageous result in the long-time experiment.

TABLE 2.-THE RELATION BETWEEN THE DIAMETER OF CAMLACHIE ORAMPEDMOLYBDENUM RODS AND THE TIME OF OREEP-RUPTURE Testing temperature: 1,0000. Stress: 5 kg./mm. gauge parameter of test pieces; 6 mm.

1, }Chuck was broken.

With the material of Nimonic 90 alloy, the life up to the rupture wasonly one hour, but when molybdenum rods were inserted, the time up tothe rupture increases proportionally to the diameter of molybdenum rodused. In the case of 5 mm. molybdenum, it was 864 hours, and in the caseof 6 mm. molybdenum, was shown such an extent of property as the chuckof the testing machine forced the stop of testing even after the elapseof 1700 hours. The reason that the rupture time was reduced in the caseof molybdenum diameter of 1 to 2 mm. is due to the fact that even smallamount of molybdenum added to the matrix of Nimonic 90 alloy, becomes akind of segregated part of coarse heterogeneous phase and the locallyconcentrated stress in this part accelerates the rupture. However, whenthe diameter of molybdenum is over 3 mm., the molybdenum and the alloylayer become to stand the stress and with the increase of diameter, therupture time is increased proportionally.

As heat-resistant alloys capable of forge-working beside Nimonic 90alloy, 25-20 alloy from the Fe base alloy, and S-816 alloy from theCo-base alloy were selected. Molybdenum rods having a diameter of 5 mm.were coated by casting with these alloys, subjected to 36 percent hotworking and made into the same specimens as above-described. The testingwas carried out in the atmosphere of air, at a temperature of 1000 C.and under the influence of stresses of 5 kg./mm. 7 kg./mm. and 10kg./mm. whereby the result shown in Table 3 and FIGURE 4 was obtained.At the side of shorter period of time, molybdenum coated with 5-816alloy showed superiority, and the molybdenum coated with Nimonic 90 and25-20 alloy showed the same degree of strength. However at the side oflonger period of time, Nimonic 90-coated molybdenum showed the mostsuperi- TABLE 3.--CREEP-RUPTURE TIME OF MOLYBDENUM CAMLACHIE CRAMPEDWITH VARIOUS HEAT-RESIST- ANT ALLOYS IN THE ATMOSPHERE OF AIR AT ATEMPERATURE OF 1,000 C.

Further investigation is added to the creep-rupture strength ofmolybdenum covered with Nimonic 90. The above-mentioned specimensconsist of three parts; the central part of pure molybdenum, thesurrounding part of molybdenum alloy layer, and the outside part ofcoating layer of Nimonic having the thickness of about 0.7 mm. Thestrength of Nimonic can, as seen from Table 3 or FIGURE 3, be neglectedat the instanced condition of the test. When assumed that the rupturestrength is only dependent upon the molybdenum part, and when 5 kg./mm.of stress is loaded on the specimens as shown in Table 4 and with dottedlines in FIGURE 5, then in the part of molybdenum diameter less than 3mm., the stress exerted on this part increases abruptly with thereduction of molybdenum diameter, but in the part of molybdenum diametermore than 4 mm., the relation of stress exerted on the molybdenum partto rupture time becomes linear as shown by the dotted straight line,regardless of how much the stress is. As the result of this, it is notproper to say that the creep-rupture strength is only dependent upon thestrength of molybdenum part in the specimens when this part is much toosmall. In reality, the rupture strength of specimens is dependent uponthe parts including both molybdenum and molybdenum alloy layer. Thisfact can be understood from the linear relationship of the strength ofmolybdenum and molybdenum alloy layer to the rupture time as shown inTable 4 and by the solid line in FIGURE 5, irrespectively of the size ofmolybdenum diameter and the value of stress.

Molybdenum having such characteristic properties has never been obtainedheretofore by any prior treating methods.

What is claimed is:

1. A composite material, capable of being subjected to hot working inair, consisting of a molybdenum core and a heat-resistant alloy coatingmaterial, said coating consisting essentially of a composition ofapproximately 18 percent by weight of cobalt, percent by weight ofchromium, 1.5% by weight of aluminum, 2% by weight of titanium and aremainder of nickel.

TABLE 4.THE RELATION BETWEEN THE STRESS EXERTED UPON MOLYBDENUM PART ANDMOLYBDENUM AND MOLYBDENUM ALLOY LAYER PARTAND THE CREEP- TIME IN THESPEOIMENS OF MOLYBDENUM COVERED WITH NIMONIC 90 Actually Actually Stressupon Stress upon Stress upon the measured measured actually actuallyparallel part 0! Diameter of diameter of diameter of measured measuredCreepspecimens Mo rods camlachie Mo+Mo diameter of diameter of rupture(kg/mm!) used (mm.) cramped alloy layer Mo Mo+Mo time (hr.)

Mo (mm.) (mm) (kg/mm!) alloy layer g./mm.=)

In FIGURE 5, triangular marks show the rupture strength of puremolybdenum tested at a temperature of 982 C..in vacua at 10, 100 and1000 hours. These points almost coincide with the solid line whichindicates that the novel coating treatment with the Nimonic 90 alloy isprotecting the molybdenum perfectly from oxidation in the atmosphere ofair at a high temperature and under the influence of stress.

The molybdenum produced according to the present invention have severalcharacteristic points. Firstly the brittle alloy layer can be workedwith molybdenum without fear of being cracked. Secondly since therecrystallization temperature is 1150" to 1200 C., it is possible tostrengthen molybdenum by strain hardening if the working is carried outat a temperature lower than the above-mentioned temperature. Thirdlythere is no need of the inactive atmospheric furnace and the vacuumfurnace for heating molybdenum. It can be freely worked in the heattreatment in the atmosphere of air. Fourthly even when imperfectlyfusion-jointed part exists locally in the carnlachie cramped state, itcan be brought to the sound state by the subsequent hot workingtreatment. Fifthly it is possible to maintain molybdenum in theatmosphere of air at a high temperature, under the influence of stressfor a long time.

2. A composite material, capable of being subjected to hot working inair, consisting of a molybdenum core and a heat-resistant alloy coating,said coating consisting essentially of a composition of approximately 20percent by weight of nickel, 3 percent by weight of iron, 20 percent byweight of chromium, 4 percent by weight of tungsten, molybdenum andcolumbium, one percent by weight of manganese, 0.2 percent by weight ofsilicon, 0.4 percent by weight of carbon and a remainder of cobalt.

3. A composite material capable of being subjected to hot working inair, consisting of a molybdenum core and a heat-resistant alloy coating,said coating consisting essentially of a composition of approximately 20percent by weight of nickel, 25 percent by weight of chromium, 1.5percent by weight of manganese, one percent by weight of silicon, 0.2percent by weight of carbon and a remainder of iron.

References Cited UNITED STATES PATENTS 2,924,004 2/1960 Wehrman 29-1983,044,156 7/1962 Whitfield 29-198 X 3,066,393 12/1962 Malagari 29-198HYLAND BIZOT, Primary Examiner.

1. A COMPOSITE MATERIAL, CAPABLE OF BEING SUBJECTED TO HOT WORKING INAIR, CONSISTING OF A MOLYBDENUM CORE AND A HEAT-RESISTANT ALLOY COATINGMATERIAL, SAID COTAING CONSISTING ESSENTIALLY OF A COMPOSITION OFAPPROXIMATELY 18 PERCENT BY WEIGHT OF COBALT, 20 PERCENT BY WEIGHT OFCHROMIUM, 1.5% BY WEIGHT OF ALUMINUM, 2% BY WEIGHT OF TITANIUM AND AREMAINDER OF NICKEL.